Statement of the Technical Field
The inventive arrangements relate to radio frequency systems and more particularly to devices used to convert between balanced signals and unbalanced signals.
Description of the Related Art
In a radio frequency device signals can be communicated as balanced or unbalanced signals. In a balanced signal scenario two conductive signal lines are provided independent of a ground. Conversely, in an unbalanced signal scenario a single signal line is provided and the signal is evaluated relative to a ground. In a radio frequency system, it is sometimes necessary to transition from a portion of the circuit which operates using balanced signals to a portion of the circuit which operates using unbalanced signals. A common example of this occurs when transitioning between a balanced antenna system (e.g., a dipole) and an unbalanced transmission line (e.g. a microstrip transmission line) used as an antenna feed. In an unbalanced transmission line such as a microstrip, there is substantial asymmetry between the conductive strip that forms the transmission line conductor as compared to the reference conductor (which in the case of microstrip would be a ground plane conductor). In such an arrangement, the current densities in each conductor are very different and they are not in differential form. Unbalanced signal transmission structures must be transformed into balanced ones in order to feed balanced radiating elements, and vice versa. A balun is a device or circuit that is used to facilitate the transition of signals between balanced signal lines and unbalanced signal lines. An additional function which is commonly performed by the balun is to provide an impedance transformation suitable to provide impedance matching as between the balanced and unbalanced structures.
When a balun is used at the output of the antenna, filtering components are commonly disposed between the balun and the remainder of the communication system so that extraneous out-of-band signals are reduced in amplitude. In this regard, it has been suggested that the functions of a balun can be combined with a tunable bandpass filter (BPF). For example, at least one system has been proposed that utilizes a split ring resonator type of element to combine the function of a balun and a BPF in one circuit.